The recovery of data from a VSB or a QAM (Quadrature Amplitude Modulated) signal at a receiver requires the implementation of three functions: timing recovery for symbol synchronization, carrier recovery (frequency demodulation) and equalization. Timing recovery is the process by which the receiver clock (timebase) is synchronized to the transmitter clock. This permits the received signal to be sampled at the optimum point in time to reduce the chance of a slicing error associated with decision-directed processing of received symbol values. Carrier recovery is the process by which a received RF signal, after being frequency shifted to a lower intermediate frequency passband, is frequency shifted to baseband to permit recovery of the modulating baseband information. Equalization is a process which compensates for the effects of transmission channel disturbances upon the received signal. More specifically, equalization removes baseband intersymbol interference (ISI) caused by transmission channel disturbances including the low pass filtering effect of the channel. ISI causes the value of a given symbol to be distorted by the values of preceding and following symbols.
For QAM signals, timing recovery is usually the first function implemented in a receiver. The timing is recovered from either the intermediate passband signal or from a near-baseband signal, i.e., a baseband signal with a carrier offset that is corrected by a carrier recovery network. In either case, timing can be established prior to baseband demodulation. The carrier recovery demodulation process is usually a two step process. First, the passband signal is demodulated to near-baseband by a frequency shifter which uses a "best guess" as to what the frequency offset is between the incoming passband signal and the desired baseband signal. This frequency shift is usually performed by analog circuits; i.e., prior to analog to digital conversion in the receiver. Next, equalization is performed on this near-baseband signal. Finally, carrier recovery is performed which removes any residual frequency offsets from the near-baseband signal to produce a true baseband output signal. This function is performed by digital receiver circuits. The equalizer is inserted between a first local oscillator which performs the shifting to near-baseband, and the carrier recovery loop network. This is because the carrier recovery process typically is a decision-directed process (as known) that requires at least a partially open "eye" which is provided by the equalizer function.
A QAM signal is represented by a two-dimensional data symbol constellation defined by Real and Imaginary axes. In contrast, a VSB signal is represented by a one-dimensional data symbol constellation wherein only one axis contains quantized data to be recovered at a receiver. Synchronous demodulation of a VSB signal has usually been accomplished with the aid of a pilot signal. The pilot signal facilitates demodulating the VSB signal to baseband in one step, typically without residual phase or frequency errors. Performing the functions of timing recovery, demodulation and equalization in the order they are performed for QAM signals does not work for VSB signals using conventional techniques. For QAM signals, several timing recovery methods are known which are independent of the frequency offset between the near-baseband signal and the baseband signal. However, it is generally accepted that frequency independent timing recovery is not feasible for VSB signals. For this reasons, in VSB systems, absolute demodulation to baseband has historically been implemented first.
In a pilot-assisted VSB system, the pilot component is injected into the baseband signal at the transmitter as a small DC offset. This DC offset generates a carrier "tone" because when the DC offset is multiplied by an alternating signal such as in the form of a high frequency cosine function, a similarly phased carrier tone results. This carrier tone can be used by a phase locked loop (PLL) in the receiver demodulator to translate the modulated VSB signal to baseband. Since the pilot represents the DC component of the VSB signal, and this DC component is in the middle of the vestigial sideband, the tone appears to be situated in the middle of modulation "noise" caused by the data itself. Normally this modulation noise is treated as unwanted signal for the pilot tracking PLL at the receiver.
The pilot can be extracted by means of a very narrow bandpass filter prior to the PLL. However, this requires a filter with such a narrow bandwidth to achieve sufficient data noise rejection that the tracking PLL is not capable of tracking all phase and frequency offsets of the incoming signal, especially when consumer grade tuners are used. Shifting the pilot component to baseband is not done easily in the digital domain. If analog circuits are used, problems of compensation for tolerances and temperature effects, for example, will have to be addressed. The pilot component also wastes power, and it is possible that a transmission channel perturbation such as a channel null will cancel the pilot. For these and other reasons, it is herein recognized as desirable to provide a system capable of demodulating a VSB signal to baseband without relying on a pilot component of the received signal.
One example of a VSB system including a pilot component is the Grand Alliance HDTV transmission system recently proposed for the United States. This system employs a VSB digital transmission format for conveying a packetized datastream, and is being evaluated in the United States by the Federal Communications Commission through its Advisory Committee of Advanced Television Service (ACATS). A description of the Grand Alliance HDTV system as submitted to the ACATS Technical Subgroup on Feb. 22, 1994 (draft document) is found in the 1994 Proceedings of the National Association of Broadcasters, 48th Annual Broadcast Engineering Conference Proceedings, Mar. 20-24, 1994.